This application claims priority under 35 U.S.C. 119 to German Patent Application No. 10102523.8, filed in the German Patent Office on Jan. 20, 2001.
The invention relates to a method for influencing the torque on at least one driving wheel of an industrial truck for the purpose of driving and/or braking it.
If the ground is in a slippery condition or loads are inappropriately distributed the problem which is frequently encountered in industrial trucks is that the driving wheel or driving wheels will slip. Furthermore, since the driven wheel frequently is also the wheel to be steered such a slip leads to poor manoeuvring characteristics. In addition, the slip causes an increase in wear to the tyres or their surfaces. The efficiency of the drive deteriorates drastically because the driving power is not turned into kinetic energy. Accordingly, this reduces the kinetic energy which can be recovered during a braking operation. If braking is done via the drive an increase in slip will naturally lead to a clear reduction of the braking torque.
The use of so-called anti-lock braking systems has generally become known from the automotive industry. The slip is determined during braking and, hence, an appropriate action onto the braking system prevents the slip from assuming excessively large rates, specifically in order to avoid a lock-up.
Although it is known for industrial trucks to employ mechanical brakes to act onto the driving wheels braking is frequently performed via the drive in industrial trucks driven by electric motors or even by hydraulic power.
Therefore, it is the object of the invention to provide a method by which the torque acting on the driving wheel of an industrial truck can be influenced, as a function of slip, for the purpose of driving and/or braking it.
The inventive method relies on an industrial truck in which the desired torque of the drive is adjusted by making a comparison of the driving rpm and a set-point adjuster. This is frequently done by the vehicle driver actuating the motor controller or a pedal, by which action a desired torque is preset for the drive depending on the wheel deflection and the respective working order, e.g. via an rpm governor which are usually possessed by such vehicles. According to the invention, the absolute value of this desired torque is reduced by a slip-dependent corrective moment. It is understood that the driving torque may have different signs which depend on whether the vehicle is in a driving or braking order during its forward or backward travel (four-quadrant operation). The corrective moment results from the respective slip that exists and is produced in the output of a slip regulator. The input of the slip regulator is acted on by a differential from a set-point of the slip and an actual value of the slip. The actual value of the slip naturally is a differential between the travelling speed of the vehicle and the circumferential speed of the driving wheel. The larger the slip is the larger will be the corrective moment so that it is sure anyway that the torque which is the respective optimum will be applied to the driving wheel or the braked wheel.
The inventive method functions safely and is independent on the various working orders. However, it requires that a desired slip be preset. If a study is made on the frictional behavior of a wheel depending on the slip the result will be that there is an optimum slip which depends on the friction pairing and will achieve most favourable conditions of friction with the ground or floor. Therefore, if possible, the desired slip will be a value which corresponds to the optimum slip, but can also differ more or less heavily from the optimum slip. The essential thing, however, is that the desired slip should not be exceeded significantly. If this is the case after all the slip regulator will act and reduce the motor torque.
The slip regulator is not used as long as the critical slip is not reached yet.
The inventive method is preferably applied to an industrial truck the drive of which is constituted by an electric motor or a combustion engine having a hydrostatic transmission with a three-phase a.c. motor being preferably provided if an electric motor is used.
In another aspect of the invention, the inventive method is preferably applied to an rpm governor which produces the desired torque for the driving motor.
In another aspect of the invention, the vehicle travelling speed of the industrial truck is determined from the number of revolutions of a non-driven wheel of the industrial truck. However, other ways of measuring speeds are possible as well. Thus, for example, an acceleration sensor may be provided with a need to integrate the vehicle acceleration to form the actual vehicle travelling speed. Further, sensors can be imagined that measure the vehicle travelling speed directly with respect to the surrounding area such as the floor, e.g. radar sensors, microwave sensors, optical or image-processing sensors, etc.
Determining the driving number of revolutions in the above described way allows to determine the slip at any time during a straight-line travel. If the slip is also to be determined during a cornering it is either required to incorporate the actual steering angle into the calculation or to perform a measurement of the speed in at least two points of the vehicle in order to determine the speed vector on the driving wheel or driving wheels in this way. In an aspect of the invention, the number of revolutions of the non-driven wheels, the steering angle of the driving wheel, the centre distance between the non-driven wheels, and the distance between the axle of the non-driven wheels and the driving wheel are used to ascertain the reference speed and this speed serves for determining the circumferential component of the driven wheel in order to establish the slip on the driving wheel.
The desired slip may be preset or determined for a certain constitution of the driving wheel. However, it is also imaginable to vary the desired slip according to the conditions around it. This is preferably done in a model-based manner by determining certain parameters during the operation of the vehicle.
If the reference speed causing a lateral drift of the vehicle is determined for steered driving wheels a detection of the axial slip may be a measure to prevent any further increase in the steering angle or even to decrease it with a view to prevent the lateral slip from rising beyond a preset degree. However, this requires a steering control.